Captured spring cabin mounting help needed

Coachgeo

Explorer
I have an LMTV. Frame/Chassis appears to have be same width as a Freightliner with an Ambulance box. Soo... I bought the box only including all the mounting hardware.


image10.jpgIMG_4419.jpg
This is Freightliner with the boxes mounting hardware still attached. You see them bolted to the chassis. They have the round disk and pad on them.


image9.jpg
The bottom of the Ambo Box has threaded studs thru a solid bar that slide into the holes on the brackets shown in above pics. Above is image that shows the studs protuding long aluminum bar across bottom of box

On the LMTV version that had a permanent box attached it used captive springs at the front and no springs at rear. Think there was two... maybe three total attach points. Only front was captive spring. Granted the "front" of that LMTV box was further back on the chassis than my first set of brackets will be when mounting this Ambulance box.


Something like this... (not from an LMTV but clearest picture I could find)

Sooo.. Armed with this information I am looking for suggestions on following plan.

1. remove some bolts from below ambulance and replace with ones long enough to handle captive spring.
2. attach brackets to my chassis matching the spacing of those on the box.... modify them if needed to use a captive spring mount.
3. Do I have to have springs above and below each bracket to allow for movement up or down.... or will just below suffice? I've seen both designs used in here.

So questions are....

. Is this captive spring idea is a good way to go? Why... Why not?
. How to determine spring and bolt sizing of captive spring?
. How to prevent corrosion issues between dissimilar metals (bolts vs Aluminum Ambo Box vs steel of brackets used on chassis. )
. looking at those mount images from Freightliner... and modification ideas for use with captive spring?
. There is 5 mounts on each side..... how many would you recommend I change to captive spring? PS- Yes I do plan to go off road that will get some twisting of chassis. NO mall / KOA campground cruiser being built with this.
 
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Coachgeo

Explorer
side note.... looks like I might have to drill an access hole in floor to get to area above the stud that would be replaced with captive spring bolt. I'll confer with the manufacture of the Ambo box on this.
 
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Ramdough

Adventurer
ABSOLUTELY! DO NOT REPLICATE THE MOUNTING SYSTEM IN THE PHOTO!!!!!

BAD, BAD, BAD Design!!!! Study a bit... You will get it!
Joaquin, good to see you around.

I know you and Victorian have a wealth of knowledge. It maybe helpful with a little more elaboration.

By the way. What mounting system did Casa Azul use?


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IdaSHO

IDACAMPER
Coachgoe, got your PM about this.

As much as Id love to comment about the captive spring setup, I honestly do not know enough about it or have enough first hand experience to comment properly.

From research Ive done over the years I have determined that I appreciate a well designed 3 or 4 point pivot torsion free setup more.

Because of that, Ill almost always opt for the 3 or 4 point pivot.
 

Coachgeo

Explorer
. Its used on literally a million Bowsers.
Only suggest it not rely on its springbolts, Supplement so its captive within guides of some configuration.
thank you for your input. What is a Bowsers?

Was thinkig earlier about the idea of a Guide. Saw a build where they put a slider plate mounted firm on box structure and only slide down along chassis. It had a piece of Nylon to rub against mounted on chassis. Anyway, it appeared to be a guide to help keep everything aligned throughout the potential range of motion. Im guessing the angle in the plate is to accomidate the portion of an ark path in which the chassis would travel when sliding during articlulating.

k5590-613slider-lateral-plate_pCrpd.jpg
 
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Ramdough

Adventurer
I have noticed that the Unimog crowd always uses a 3-4 pivot mount. The Unimog frame flexes like crazy though. Many other truck have huge flex as well.

On the US Millitary expansible van chassis the captive springs are used from the factory.

Duckworthe has only a rigid mount with no problems. His claim is that the LMTV has a very rigid frame design and it does not twist like a Unimog. He also drives his truck hard.

Basically there is a range of movement depending on what vehicle you pick. I would argue that every vehicle flexes more than you want to transfer to your camper.

IMHO, I would at minimum use springs, but the properly designed 3-4 pivot systems are probably the ultimate protection.

I have not picked what system I want to use yet for my future camper. The spring systems seem way easier to fabricate. Easier is not always better when you are sinking this kind of money into a project.

I look forward to seeing what you decide Coach.
 

Ramdough

Adventurer
thank you for your input. What is a Bowsers?

Was thinkig earlier about the idea of a Guide. Saw a build where they put a slider plate mounted firm on box structure and only slide down along chassis. It had a piece of Nylon to rub against mounted on chassis. Anyway, it appeared to be a guide to help keep everything aligned throughout the potential range of motion. Im guessing the angle in the plate is to accomidate the portion of an ark path of the chassis would move when articlulating.

View attachment 395377
Looks like there is also a material between the top and bottom frames as well.


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Coachgeo

Explorer
I have noticed that the Unimog crowd always uses a 3-4 pivot mount. The Unimog frame flexes like crazy though. Many other truck have huge flex as well.

On the US Millitary expansible van chassis the captive springs are used from the factory.

Duckworthe has only a rigid mount with no problems. His claim is that the LMTV has a very rigid frame design and it does not twist like a Unimog. He also drives his truck hard. ....
Yes but I believe like all other LMTV mounted beds his box base contains a strong 6" channel Iron that runs length of a bottom of camper box... and it is that is bolted to the chassis of the truck . This design you would think makes the truck chassis even more stiff. I believe the m1079's box has this 6" channel Iron on its bottom too though... with solid mount to truck chassis in back and spring mount in front.
 

Coachgeo

Explorer
Sorry, (Freekin 'Yanks :)
Bowser is a tanker truck.
If you are not feeling crafty, you can buy these type mountings off the shelf.
thank you. What would I look for (Internet search keywords) to help me find sources for purchasing these. Or if you have any direct sources you can point me to I'de appreciate that.
 

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dwh

Tail-End Charlie
Pretty sure Jack was saying to go with spring mount and not to use the rubber donut mounting system.

The rubber donut system creates a ridgid coupling between box and frame, so if the frame flexes it will twist the box unless the box is strong enough to resist the twist, in which case it strengthens the frame.

Fine for an on-road truck, not good for off-road.

The captive spring setup allows the frame to twist under the box and as long as the frame twist isn't too much, it won't twist the box. Lots of Aussie Canters have that sort of mounting.

Most I've seen have the box attached ridgid at the rear end of the frame, with the spring mounts the rest of the way forward. That allows the box and frame to separate while still preventing the box from shifting fore and aft.

And then there are side plates to prevent side to side shift.



Toner is the expert:

http://m.trailer-bodybuilders.com/distributors-upfitters/tips-truck-frames



“The body should not be rigid at the front,” he said. “It is better to be mounted solidly at the rear and float at the front. Avoid high moment areas for the mounting brackets. Use existing frame holes when possible.”
 
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